In a dialysis machine of the above-mentioned type, one or more cartridges containing sodium bicarbonate powder, sodium chloride or other salts are used. The salt is dissolved by the introduction of water into the cartridge and removal of concentrate from the cartridge. The concentrate is used to prepare the intended dialysis solution.
The composition of the dialysis solution is determined in the dialysis machine by measuring the conductivity of the prepared solution and regulating dosage pumps for each of the various concentrates. It is now common to use two different concentrates; a B-concentrate comprising only bicarbonate from the above-mentioned bicarbonate cartridge, and an A-concentrate comprising the remaining components in concentrated form, for example to a 37-fold concentration. Other combinations of concentrates also exist, such as the B-concentrate, which besides bicarbonate, may also comprise sodium chloride. Alternatively, the B-concentrate can be divided into two parts comprising bicarbonate and sodium chloride, respectively, whereby the A-concentrate comprises the remaining components in a more concentrated form.
By preparing the bicarbonate concentrate and, where appropriate, the sodium chloride concentrate on demand in the dialysis machine, an advantage is realized in that the bicarbonate concentrate remains stable until it is used in a dialyser connected to the dialysis machine.
If a dialysis solution is prepared in advance, which is also now a common procedure, the risk exists that the bicarbonate can decompose to carbon dioxide and carbonate. This implies that the pH value for the solution rises and the risk of precipitation of the calcium carbonate arises during mixing to the prepared dialysis solution. This can affect the final composition of the dialysis solution (reduction of the calcium ion concentrate) as well as creating problems of silting up of conduits and components in the dialysis machine due to the deposition of calcium carbonate. For these reasons, the bicarbonate cartridge as described above has attained widespread use.
As mentioned above, the dialysis solution is prepared by mixing the two concentrates with water. The mixing process is regulated by conductivity meters which control various dosage pumps. Conductivity meters are, however, sensitive to possible incorporation of gas bubbles in the solution. Thus, the conductivity meters are generally preceded by gas separators whereby more accurate, less fluctuating measuring values can be obtained.
The dialysis machine is provided with a monitoring system which is separated from the regulating system and serves to emit alarm signals should error situations arise. In the above-mentioned dialysis machine; namely, GAMBRO AK 100, monitoring of the dosage of concentrate occurs by monitoring the number of revolutions of the dosage pumps. If the number of revolutions differs too greatly from an expected value, an alarm signal is emitted.
When using the above-mentioned bicarbonate cartridge containing dry bicarbonate powder, it is necessary that the powder be wetted with water prior to use. This takes place in a particular "priming step". Water is introduced into the cartridge at its upper end at the same time that a substantial vacuum is applied to the lower end of the cartridge. Water thus fills substantially the entire cartridge in less than a minute.
When a sensor positioned downstream of the bicarbonate cartridge detects that primarily concentrate is flowing from the cartridge, the sensor indicates that the priming step is complete. A valve then switches over the machine so that the substantial vacuum is terminated. This sensor can be the above-mentioned conductivity meter.
During the priming step, a small quantity of air or gas is normally trapped at the upper end of the cartridge. This quantity of gas does not, however, normally affect the functioning of the cartridge. In the above-mentioned European Patent No. 278,100, various methods are described for removing this quantity of gas before the dialysis treatment commences, i.e. during the priming step.
However, it sometimes occurs that the trapped gas in the upper region of the cartridge increases in volume during the dialysis treatment. If the trapped volume of gas becomes so great that a considerable quantity of gas passes out through the outlet of the cartridge and reaches the conductivity meter, an alarm is raised. Furthermore, it will be appreciated that the normal functioning of the cartridge is greatly affected if far too great a volume of gas is present in the cartridge. Normally, it is preferred that the water level always remains above the salt particle level in the cartridge.
The above-mentioned condition with increasing volume of gas can be attributed to several causes. One possible cause is leakage in the connection between the upper or lower ends of the cartridge and the dialysis machine. The dialysis machine normally maintains a small vacuum in the cartridge. Another such cause can be that gas bubbles accompany the water which enters the cartridge and thereafter become separated in the cartridge. The principal cause would seem, however, to be gas formation in the cartridge, such as formation of carbon dioxide gas.
It has been observed that the above-mentioned problem is exacerbated at higher ambient temperatures, which is probably due to the decomposition of bicarbonate to carbon dioxide and carbonate.
Since the cartridge circuit is closed, there is no other route for the gas to flow than through the outlet from the cartridge, something which can activate the above-mentioned conductivity alarm. In order to deal with such an alarm situation, it is then necessary to remove the bicarbonate cartridge and to insert a new cartridge into the system, whereafter the machine has to be restarted with a priming step and subsequent stabilising steps, something which can take a long period of time. During this time, the dialysis treatment has to be interrupted.
It has also been observed that the conductivity signal from the conductivity meter, despite the preceding gas situation, fluctuates greatly, particularly at high ambient temperatures. In extreme cases, such as at high temperatures, these fluctuations are so great that the alarm limit is exceeded.
It is noted that Japanese Patent No. 55115819 describes a method of degassing the water, and also possibly the concentrates, prior to mixing at volumetric dilution in order to avoid problems relating to air bubbles being formed during heating.